-- Mode
-- 重力模式表，在main中可更改重力模式
Mode = {
earth = {gravity = 9.81},
space = {gravity = 0,tick = function(world)
    for k,body in pairs(world.objects) do
        body:applyForce(ObjectForce.calcGravity(world,body) * 100)
    end
end},
mars = {gravity = 3.72},
moon = {gravity = 1.63},
AntiForce = {gravity = -9.81},
trrx = {gravity = 450},
noair = {gravity = 9.8},
brownmode = {gravity = 9.82,tick = function(world)
    for k,body in pairs(self.objects) do
        body:applyForce(vec2((math.random() * 2 - 1) * 15000,(math.random() * 2 - 1) * 15000))
    end
end}
}